The present invention relates generally to the field of monitoring systems including, but not limited to, a switch status monitoring system arranged for operating in both a supervised mode and an unsupervised mode.
In building access control systems it is important to sense the position, open or closed, of the doors monitored by the system. Two approaches to door position monitoring systems are respectively referred to as unsupervised monitoring systems and supervised monitoring systems. Each of the unsupervised monitoring system and the supervised monitoring system use simple, low cost normally open or normally closed switches that change state when the monitored door moves.
An unsupervised door monitoring system has an input representing the two states door open (active) and door closed (clear) as typically provided by an output of a reed switch arranged for opening and closing with the monitored door. Unsupervised door monitoring systems are typically implemented as a digital logic unit. The digital logic unit evaluates the input and provides a digital value (a xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d) corresponding to the door open and door closed states typically to an application processing unit associated with an overall building management system.
A supervised door monitoring system has an input representing typically four or more possible states. Two of the typical four states correspond to the door open (active) and the door closed (clear) states as provided by an associated reed switch. These two states are also often referred to as normal operation. The remaining two states correspond to a short state and an open state. The short state represents a condition where the switch or the wiring connection has developed a short circuit as a result of wiring failure or intentional tampering. The open state represents a condition where the switch or the wiring connection has developed an open circuit. The input in a supervised door monitoring system is an analog electrical signal the magnitude o f which represents at least one of the four possible states. The supervised door monitoring system, therefore, is typically implemented using an analog-to-digital converter or an analog window comparator. The output of either of the analog-to-digital converter or the analog window comparator is typically a count value (ranging from 0 to n counts). The count value, representative of the door status, is provided to an application processing unit associated with an overall building management system or access control unit.
To convert a system implemented as an un supervised door monitoring system to a supervised door monitoring system and vise versa requires not only changing the relatively low cost switch, but also requires changing the associated processing unit. Also, because specialized hardware is required for providing either supervised or unsupervised operation, point-by-point selection of operating modes for particular monitored switch system s is not possible.
Thus, there is a need for an improved door monitoring system that allows flexibility in switch type as well as digital and/or analog or xe2x80x9cnxe2x80x9d state monitoring.